As mentioned above, the photometric system of the invention can be used for spectrophotometric purposes, or for calibrating optical attenuators. The use of optical attenuators of known attenuation in spectrophotometric systems, and the like, requires that the attentuation of such attenuators be known to a precise degree. However, the calibrated accuracy of optical attenuators, such as neutral density glass filters, interference-type filters, comb filters, screen filters, and the like, has been limited in the prior art, especially at high optical densities. Accordingly, there is a pressing need to provide greater accuracy in the measurement of the attenuation of such attenuators, and one purpose of the system of the present invention is to provide a means for fulfilling that need.
Accordingly, one objective of the present invention is to provide a photometric system for precisely calibrating optical attenuators, and the like; and another objective of the invention is to provide such a photometric system which may be used for spectrophotometric purposes.
The photometric system of the invention, as mentioned above, includes a time-division optical attenuator which is used to attenuate a light beam within the system with a high degree of accuracy. The photometric system of the invention is such that non-linearities are eliminated, so that the transmittance (T) of a sample, or the calibration of an optical attenuator, may be achieved with a high degree of accuracy.
The photometric system of the invention, when used to calibrate an optical attenuator, for example, first measures the light passing through a reference attenuator. The optical attenuator to be calibrated is then introduced into the system, and the light incident on the latter attenuator is changed precisely by any ratio of integers by the time-division attenuator included in the system, so as to return the light output of the system to nearly its original reference level. As mentioned above, since the time-division optical attenuator can make changes in the light intensity which are theoretically without error, the accuracy of the measurement of the attenuator being calibrated depends only on the accuracy to which the small difference between the reference and test light outputs of the system can be measured.
For example, if an attenuator to be calibrated passes 9.8% of the incident light, the optical division attenuator is changed from the reference to the test mode of the system to pass exactly ten times the amount of light, so that the accuracy of the calibration will depend upon how accurately one can measure the 2% change in the output light signal levels between the reference and the test conditions.